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Wednesday, 31 July 2013

Ice-free winters during the Pliocene

Year-round
ice-free conditions across the surface of the Arctic Ocean could
explain why Earth was substantially warmer during the Pliocene Epoch
than it is today, despite similar concentrations of carbon dioxide in
the atmosphere, according to new research carried out at the
University of Colorado Boulder.

In
early May, instruments at the Mauna Loa Observatory in Hawaii marked
a new record: The concentration of carbon dioxide climbed to 400
parts per million for the first time in modern history.

The
last time researchers believe the carbon dioxide concentration in the
atmosphere reached 400 ppm -- between 3 and 5 million years ago
during the Pliocene -- Earth was about 3.5 to 9 degrees Fahrenheit
warmer (2 to 5 degrees Celsius) than it is today. During that time
period, trees overtook the tundra, sprouting right to the edges of
the Arctic Ocean, and the seas swelled, pushing ocean levels 65 to 80
feet higher.

Scientists'
understanding of the climate during the Pliocene has largely been
pieced together from fossil records preserved in sediments deposited
beneath lakes and on the ocean floor.

"When
we put 400 ppm carbon dioxide into a model, we don't get as warm a
planet as we see when we look at paleorecords from the Pliocene,"
said Jim White, director of CU-Boulder's Institute of Arctic and
Alpine Research and co-author of the new study published online in
the journal Palaeogeography, Paleoclimatology, Palaeoecology. "That
tells us that there may be something missing in the climate models."

Scientists
have proposed several hypotheses in the past to explain the warmer
Pliocene climate. One idea, for example, was that the formation of
the Isthmus of Panama, the narrow strip of land linking North and
South America, could have altered ocean circulations during the
Pliocene, forcing warmer waters toward the Arctic. But many of those
hypotheses, including the Panama possibility, have not proved viable.

For
the new study, led by Ashley Ballantyne, a former CU-Boulder doctoral
student who is now an assistant professor of bioclimatology at the
University of Montana, the research team decided to see what would
happen if they forced the model to assume that the Arctic was free of
ice in the winter as well as the summer during the Pliocene. Without
these additional parameters, climate models set to emulate
atmospheric conditions during the Pliocene show ice-free summers
followed by a layer of ice reforming during the sunless winters.

"We
tried a simple experiment in which we said, 'We don't know why sea
ice might be gone all year round, but let's just make it go away,' "
said White, who also is a professor of geological sciences. "And
what we found was that we got the right kind of temperature change
and we got a dampened seasonal cycle, both of which are things we
think we see in the Pliocene."

In
the model simulation, year-round ice-free conditions caused warmer
conditions in the Arctic because the open water surface allowed for
evaporation. Evaporation requires energy, and the water vapor then
stored that energy as heat in the atmosphere. The water vapor also
created clouds, which trapped heat near the planet's surface.

"Basically,
when you take away the sea ice, the Arctic Ocean responds by creating
a blanket of water vapor and clouds that keeps the Arctic warmer,"
White said.

White
and his colleagues are now trying to understand what types of
conditions could bridge the standard model simulations with the
simulations in which ice-free conditions in the Arctic are imposed.
If they're successful, computer models would be able to model the
transition between a time when ice reformed in the winter to a time
when the ocean remained devoid of ice throughout the year.

Such
a model also would offer insight into what could happen in our
future. Currently, about 70 percent of sea ice disappears during the
summertime before reforming in the winter.

"We're
trying to understand what happened in the past but with a very keen
eye to the future and the present," White said. "The piece
that we're looking at in the future is what is going to happen as the
Arctic Ocean warms up and becomes more ice-free in the summertime.

"Will
we continue to return to an ice-covered Arctic in the wintertime? Or
will we start to see some of the feedbacks that now aren't very well
represented in our climate models? If we do, that's a big game
changer."